Add support for regularizing truncated diagonal entries

This adds support for regularization of truncated diagonal
entries in `BoxConstrainedQuadraticFunctionalConstrainedLinearization`
If this is enabled, diagonal entries of truncated rows are set
to 1. In case of nested matrices this is applied recusively,
such that only the truncated scalar diagonal entries of nontrivial
diagonal blocks are modified. This can be handy if the truncated
linearized problem should be treated by a linear solver.
By default this is disabled, such that linear solvers for the
linearized problem have to be robust with respect to singular
problems due to truncation.
4 jobs for feature/regularize-truncated-diagonal in 11 minutes and 30 seconds (queued for 5 seconds)
Status Name Job ID Coverage
  Test
passed dune:git clang C++17 #160775

00:05:22

passed dune:git clang C++17 with examples #160777

00:01:48

passed dune:git gcc-8 C++17 #160776

00:07:22

passed dune:git gcc-8 C++17 with examples #160778

00:11:29